Chapter 3 Water’s Density à Water is less dense as a solid than it is as a liquid. - Unlike most fluids, water expands as it solidifies instead of contracting. - At temperatures above 4°C, water acts like other liquids. However when water reaches 4°C it begins to freeze because its H20 molecules are no longer moving fast enough to break their hydrogen bonds. à When water gets down to 0°C it becomes locked in a crystalline lattice, where the water molecule is bonded to up to 4 other molecules. à When ice begins to melt, some of the hydrogen bonds begin to break, so the water molecules can begin to move closer together while in the ice state. à Ice is approximately 10% less dense than water at 4°C. à Ice floats on the cool water below. à This unique property has consequences in life - If ice sank, all ponds, lakes, and oceans would freeze solid and even in warm weather, only a few centimeters of the ocean would thaw. - However, because of this odd density, the surface layer of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.
Water as the Solvent of Life à A solution is a liquid that is a completely homogeneous mixture of two or more substances; a solvent is a dissolving agent; the substance dissolved is the solute. An aqueous solution is when water is the solvent. Ex. Sugar water is the solution - water is the solvent and sugar is the solute. (This is an aqueous solution!) à Water is NOT the universal solvent, but it is very versatile due to the polarity of its water molecules. It readily forms hydrogen bonds with charged and polar covalent molecules. Ex. A crystal of salt (NaCl) is placed in water, the Na+ cations interact with the partial negative charges of the oxygen regions of the water molecules. The Cl- anions then interact with the partial positive charges of the hydrogen regions of the water molecules. à Large molecules can dissolve in water if the have ionic and polar regions. à A substance that has an affinity for water (or is water-loving) is called hydrophilic. The substances are dominated by ionic or polar bonds. Some hydrophilic substances can’t dissolve because their molecules are too large. à A substance that has no affinity for water (or is water-fearing) is called hydrophobic. These substances are nonionic and have non-polar covalent bonds. Water cannot form hydrogen bonds with these substances because there are no consistent regions with partial or full charges. Hydrophobic molecules are the major ingredients of cell membranes. à Chemical reactions depend on collisions of molecules and therefore on the concentrations of solutes in aqueous solutions. à The unit measurement of number of molecules is moles and is represented Avogadro’s number (6.02 * 1023) which is equal to the molecular weight of a substance but scaled up from Daltons to grams. à the concentration of a material in solution is called its molarity.
Water Molecules Dissociate and Form Ions:
Very few molecules are dissociated at any given time.
There is an equilibrium between the molecules and the ions (meaning the rates are the same, not the quantities are the same!!!) where the water molecule dissociates into a hydrogen ion and a hydroxide ion.
H20 ßà H+ + OH-
This equation means that a proton has separated from the water molecule (hydrogen ions are simply protons).
At this equilibrium the concentration of hydrogen and hydroxide ions is 10−7M each.
The Dissociation of Water is the Basis for pH:
pH is a logarithmic scale of how basic or acidic an AQUEOUS solution is (Rocks don’t have a pH).
The scale runs from 0-14:
0-7 means an acidic solution (more hydrogen ions)
7 exactly means a neutral solution (equal concentrations of both ions)
7-14 means a basic solution (more hydroxide ions)
an acid lowers the amount of OH- or increases the amount of H+.
a base lower the amount of H+ or increases the amount of OH-.
STRONG acids and bases are those that dissociate completely in water; WEAK acids and bases only partially dissociate.
the product of the concentrations of OH- and H+ will ALWAYS be
Water Moderates Temperature:
Heat vs. Temperature Heat: total kinetic energy (energy of motion) due to molecular motion Temperature: the intensity of heat due to the average kinetic energy of molecules [*speed of molecules increases, temperature increases] Temperatures to know: - 0 °C = freezing - 100 °C = boiling - 37 °C = human body - 20-22 °C = room - 4 °C = densest of any water form (water molecules expand before locking together)
Water has a high specific heat: the amount of energy absorbed or lost to raise 1 g of a substance by 1°C (it takes more energy to disrupt hydrogen bonds) Examples: water absorbs a lot of heat but doesn’t change temperature; warm water warms cooler air in the winter; coastal areas have more stable temperatures; living things resist temperature change because they are mostly water)
Heat of Evaporation: the amount of energy needed for 1 g of a substance to be converted from liquid to gas (because of hydrogen bonds) *Evaporative Cooling: as water evaporates, the surface left behind cools - Temperature moderation in lakes and ponds - Perspiration
Water’s Density
à Water is less dense as a solid than it is as a liquid.
- Unlike most fluids, water expands as it solidifies instead of contracting.
- At temperatures above 4°C, water acts like other liquids. However when water reaches 4°C it begins to freeze because its H20 molecules are no longer moving fast enough to break their hydrogen bonds.
à When water gets down to 0°C it becomes locked in a crystalline lattice, where the water molecule is bonded to up to 4 other molecules.
à When ice begins to melt, some of the hydrogen bonds begin to break, so the water molecules can begin to move closer together while in the ice state.
à Ice is approximately 10% less dense than water at 4°C.
à Ice floats on the cool water below.
à This unique property has consequences in life
- If ice sank, all ponds, lakes, and oceans would freeze solid and even in warm weather, only a few centimeters of the ocean would thaw.
- However, because of this odd density, the surface layer of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.
Water as the Solvent of Life
à A solution is a liquid that is a completely homogeneous mixture of two or more substances; a solvent is a dissolving agent; the substance dissolved is the solute. An aqueous solution is when water is the solvent.
Ex. Sugar water is the solution - water is the solvent and sugar is the solute. (This is an aqueous solution!)
à Water is NOT the universal solvent, but it is very versatile due to the polarity of its water molecules. It readily forms hydrogen bonds with charged and polar covalent molecules.
Ex. A crystal of salt (NaCl) is placed in water, the Na+ cations interact with the partial negative charges of the oxygen regions of the water molecules. The Cl- anions then interact with the partial positive charges of the hydrogen regions of the water molecules.
à Large molecules can dissolve in water if the have ionic and polar regions.
à A substance that has an affinity for water (or is water-loving) is called hydrophilic. The substances are dominated by ionic or polar bonds. Some hydrophilic substances can’t dissolve because their molecules are too large.
à A substance that has no affinity for water (or is water-fearing) is called hydrophobic. These substances are nonionic and have non-polar covalent bonds. Water cannot form hydrogen bonds with these substances because there are no consistent regions with partial or full charges. Hydrophobic molecules are the major ingredients of cell membranes.
à Chemical reactions depend on collisions of molecules and therefore on the concentrations of solutes in aqueous solutions.
à The unit measurement of number of molecules is moles and is represented Avogadro’s number (6.02 * 1023) which is equal to the molecular weight of a substance but scaled up from Daltons to grams.
à the concentration of a material in solution is called its molarity.
Water Molecules Dissociate and Form Ions:
The Dissociation of Water is the Basis for pH:
Water Moderates Temperature:
Heat vs. Temperature
Heat: total kinetic energy (energy of motion) due to molecular motion
Temperature: the intensity of heat due to the average kinetic energy of molecules [*speed of molecules increases, temperature increases]
Temperatures to know:
- 0 °C = freezing
- 100 °C = boiling
- 37 °C = human body
- 20-22 °C = room
- 4 °C = densest of any water form (water molecules expand before locking together)
Water has a high specific heat: the amount of energy absorbed or lost to raise 1 g of a substance by 1°C (it takes more energy to disrupt hydrogen bonds)
Examples: water absorbs a lot of heat but doesn’t change temperature; warm water warms cooler air in the winter; coastal areas have more stable temperatures; living things resist temperature change because they are mostly water)
Heat of Evaporation: the amount of energy needed for 1 g of a substance to be converted from liquid to gas (because of hydrogen bonds)
*Evaporative Cooling: as water evaporates, the surface left behind cools
- Temperature moderation in lakes and ponds
- Perspiration